A fundamental question in the aging field is whether the age-related decline in tissue-specific adult stem cell function is reversible. Focused on the gut, our preliminary studies suggest that intestinal stem cell (ISC) numbers are reduced in old mice and humans and that intestinal crypts isolated from old mice are less functional in an in vitro organoid assay of ISC function. We also find that calorie restriction (CR) reverses the effects of aging on ISCs. In the mammalian intestine, a majority of ISCs express Lgr5 and are adjacent to Paneth cells, which constitute a component of the stem cell cellular neighborhood or niche. We have recently demonstrated that CR in young mice augments ISC function by reducing mechanistic target of rapamycin complex 1 (mTORC1) signaling in Paneth cells, and that these effects of CR can be mimicked by rapamycin (an mTORC1 inhibitor). This interaction between Paneth cells and ISCs is mediated by expression in Paneth cells of bone stromal antigen 1 (Bst-1), an ectoenzyme that produces the paracrine factor cyclic ADP ribose (cADPR). Identification of the mechanistic steps in this process through the three aims of this proposal wil increase our understanding of how CR protects an organism against the age-related decline in tissue function. Specifically, we will test the hypotheses that induction of niche Bst-1 by CR and rapamycin boosts ISC function in old mice (Aim 1); that the transcription factor PPAR-gamma mediates this response in Paneth cells (Aim 2); and that cADPR-activated signaling mediates this response in ISCs (Aim 3).